Summary: The Plant Cell, Vol. 8, 1095-1105, July 1996 O 1996 American Society of Plant Physiologists
RESEARCH ARTICLE
Expression of the Pseudomonas syringae Avirulence
Pmtein AvrB in Plant Cells Alleviates Its Dependence
on the Hypersensitive Response and Pathogenicity (Hrp)
Secretion System in Eliciting Genotype-Specific
Hypersensitive Cell Death
Suresh Gopalan,'~' David W. Bauer,b9'James R. Alfano,b Amy O. Loniello,b Sheng Yang He,a
and Alan Collmerbi2
a Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824-1312
b Department of Plant Pathology, Cornell University, Ithaca, New York 14853-4203
The nonpathogenicbacteria Pseudomonas fluorescens and Escherichia coli can elicit a genotype-specifichypersensi-
tive response (HR) in plants if they express both the HR and pathogenesis (Hrp) protein secretion system and the HrpZ
harpin from I?syringae pv syringae 61 and a c! syringae avirulence (avr) gene whose presence is recognized by a corre-
sponding disease resistance gene in the plant. We have found that the recognition event appears to require transfer of
the Avr protein into the plant cell. Elicitation of a genotype-specific HR was observed with avrB+ I? fluorescens in soy-
bean and Arabidopsis plants carrying resistance genes RPG7 and RPM7, respectively, and with avrPto+E. coli in tomato
plants carrying resistance gene PTO, but only if the Hrp secretion system, HrpZ, and the appropriate Avr proteins were
produced in the same bacterial cell. The failure of avrB hyperexpression and exogenousAvrB or HrpZ to alleviate these
requirementsin soybean and Arabidopsis suggests that the site of AvrB action is not in the bacterialcell or plant apoplast.